The differentiation between structure and process in a system — between, roughly speaking, the fixed and the variable parts of the system — is generally recognized [Wand and Weber, 1990, pp. 125-126; Weinberg and Weinberg, 1979, pp. 122-135]. The relationship between structure and process is that

structure constrains process

. Changes made to a system that are limited to its process components tend to be low in cost because the changes remain local. Changes to a system’s structure tend to be high in cost because they alter the constraints on all the system’s processes that access the changed portion of the structure.